This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The experiment aims at providing experimental data for an investigation of photo-excitation processes in molecular single-crystals. The initial photo-excitation processes occur on extremely short time-scales (femto-/picosecond time domain) and have been in the focus of scientific investigations due to their possible applications, e.g. as optical switches. Following optical excitation several species can be formed in a single crystal, some of which live up to some milliseconds. We propose to carry out time-resolved Laue diffraction following photo-excitation for varying laser intensities on metal-ligand molecular crystals. These compounds have extraordinary characteristics in photo-producing electron-spin flipped triplet states. One direct application of this mechanism is built in the functional design of optical light emitting diodes. The proposed experiments carried out at different temperatures will give an additional inside into the photo-induced structural mechanism. This should provide information about the structural dynamics underlying the population and decay mechanism of the photoexcited species in crystals clarifying fundamental questions of heat transfer and population of structural states which can actively be used for light-emitting application. As sample we have selected Rh2(1,8-diisocyano-p-menthane)4(PF6)2&#903;CH3CN (Rh-dimen), a compound that is known to produce exited states in the single crystal with varying lifetimes in the 100 ns-[unreadable]s timescales at temperatures from 15 K to room temperature. Excited state populations of 1.9% (532 nm excitation) and 2.5% (355 nm) have already been refined in experiments with microsecond time resolution. From ultrafast optical experiments (in house research) we expect populations of the light-active triplet state of up to 5% for 400 nm excitation with 100 fs laser pulses.We propose to first investigate in the response of the crystal to higher laser intensities and then carry out temperature dependent measurements.